a. Field of Invention
This invention pertains to the receiver section of a digital modem and more particularly to an improved modem section for receiving and decoding quadrature amplitude modulated data signals.
b. Description of the Prior Art
Digital data signals are frequently encoded using a quadrature amplitude modulation (QAM) scheme for high speed and reliability. Such signals are characterized by an in-phase and a quadrature component and may be illustrated as points on a Cartesian coordinate system with the above components defining the orthogonal coordinates of the points. The points in each of the four quadrants form identical patterns in virtually all the signal constellations (or eye patterns) presently in use. Yet in all the present modem receiver sections, provisions are made for all the points of signal constellations regardless of their inherent symmetry leading to unduly complex implementations. This is especially true of microprocessor-based modem receivers in which complex look-up tables are used by decoding, as well as for various error correction functions (described more fully below). These look-up tables must provide proper values for all the points of the constellation. In addition, various manipulations on the signal components are performed (such as additions and multiplication). Since at least one of the components of all the points outside the first quadrant is negative, special provisions must be made for handling negative numbers, leading to further complications.